fix: consolidate CMS page seed scripts and fix 3 bugs

- Fix `ContentPage.store_id is None` (Python identity check, always
  False) → use `.is_(None)` for proper SQLAlchemy NULL filtering
- Create pages for ALL platforms instead of only OMS
- Merge create_platform_pages.py into create_default_content_pages.py
  (5 overlapping pages, only platform_homepage was unique)
- Delete redundant create_platform_pages.py
- Update Makefile, install.py, and docs references

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/getting-started/platform-homepage-quick-start.md

@@ -11,10 +11,10 @@ Quick reference for setting up and customizing the platform homepage and content
 Run the seeder script to create all default platform pages:
 
 ```bash
-python scripts/seed/create_platform_pages.py
+python scripts/seed/create_default_content_pages.py
 ```
 
-This creates:
+This creates (for all platforms):
 - ✅ Platform Homepage (modern template)
 - ✅ About Us
 - ✅ FAQ
@@ -253,7 +253,7 @@ Result in header: `About | FAQ | Contact`
 
 **Fix:** Run seeder script
 ```bash
-python scripts/seed/create_platform_pages.py
+python scripts/seed/create_default_content_pages.py
 ```
 
 ### Navigation menu is empty